The present invention relates to plasma processing methods and apparatus, and more particularly to a plasma processing method and apparatus fit for processing a specimen, such as a semiconductor substrate (hereinafter called a "wafer"), by generating a plasma using a microwave and a high-frequency wave in the frequency range of 10 to 100 MHz.
In the case of apparatus in general for generating a plasma by introducing a microwave into a process chamber, the uniformity of plasma density distribution above a wafer surface to be treated is particularly important in securing a desired uniformity of processing, such as etching. In order to deal with this problem, there has been proposed a method of radiating a microwave in a ring-like shape from the top surface of a process chamber to generate a ring-shaped plasma, so that uniform plasma distribution is obtained on the surface of a wafer, as mentioned in, for example, Document A, "The Japan Society of Applied Physics, 1994, Autumn, 19p-ZV-4" or Document B, "The Japan Society of Applied Physics, 1994, Autumn, 19pZV-6." This is because the plasma tends to diffuse as it is moved from its originating place in the direction of the wafer. Although a desired uniformity of the etching rate itself is ultimately required, it may be desirable for the apparatus also to have a density distribution adjusting means, since the plasma density distribution is concave or convex, rather than a uniform distribution.
Moreover, another means for generating a ring-shaped plasma is described in, for example, Japanese Patent Laid-Open No. 112161/1994.
On the other hand, an inductively coupled RF (high frequency) plasma source as representatively disclosed in Japanese Patent Laid-Open No. 79025/1991, has been in frequent use recently as a plasma source for CVD and etching. This plasma source has made possible not only a high density (10.sup.11 -10.sup.12 cm.sup.-3) but also a low-pressure (1-10 mTorr) action equivalent to what is offered by a microwave ECR plasma source, though it is compact in construction. Even in this system, however, it is still required to provide a definite means for securing a desired plasma uniformity and to solve a problem arising from foreign matter produced by wall-surface sputtering, which will be described below, as in the case where a microwave is used.
First, a description will be given of a problem concerning microwave plasma uniformity herein. The ring-shaped plasma radiation means according to the above-described Documents A and B has been arranged so that it is fit for use in producing a uniform plasma, but the ultimate plasma density thus attained thereby remains at a low level, such as 3-10.times.10.sup.10 cm.sup.-3 and still fails to reach the following level which is industrially required 3-10.sup.-11 cm.sup.-3 or greater.
This is considered attributable to the fact that, in both the cases mentioned above, the absorption efficiency of a microwave has not been optimized, because a local magnetic field produced by a permanent magnet is employed at an outlet for the microwave, nor has the design of a microwave transmission path for large electrical power to be transmitted. According to the above-referenced Document A, the introduction of a complicated three-dimensional structure into the plasma processing chamber may cause foreign matter to be produced. Japanese Patent Laid-Open No. 112161/1994 describes an arrangement in which a coaxial waveguide is opened in a tapered shape, which results in rendering a microwave radiating portion large-sized.
A description will subsequently be given of a problem concerning plasma uniformization and foreign matter control in the case of a high-frequency wave.
The aforementioned system (Japanese Patent Laid-Open No. 79025/1991) has presented problems, including the need for the inner surface of a vacuum chamber to be scraped down as a result of sputtering due to ion bombardment, thus increasing not only the production of foreign matter, but also increasing the frequency at which parts need to be replaced in the vacuum chamber; and, in addition, there is lowering of the plasma uniformity as the plasma tends to concentrate at the center of the chamber and so forth.
In this case, the induction coil placed outside the vacuum chamber undergoes partial electrostatic coupling with the plasma, rather than the intended inductive coupling therewith, and ions are accelerated by this electrostatic coupling toward the inner surface of the vacuum chamber, whereby the sputtering of the inner surface is said to occur. Consequently, an attempt was made to remove the electrostatically coupled component by introducing an electrostatic shield, called a Faraday shield, between the induction coil and the vacuum chamber so as to suppress the sputtering. However, the effect of this arrangement is not perfect and there still remains a scraping problem because of the sputtering: (e.g., Y. Hikosaka et al., "Free Radicals in an Inductively Coupled Etching Plasma," Jpn. J. Appl. Phys. Vol 133 (1994) pp 2,157-2,163 Part 1. No. 4B, April 1994).
In the system disclosed in Japanese Patent Laid-Open No. 79025/1991, further, a plasma generating area over the whole top surface of the chamber and the aforementioned plasma diffusion effect have been combined to cause the plasma to be centralized.
Moreover, another problem pertaining to the plasma processing apparatus (Japanese Patent Laid-Open No. 79025/1991) has arisen from the ignitability and stability of plasma. When the induction system is used to ignite a plasma, it is necessary to form the top surface of the process chamber with dielectric material so as to introduce the magnetic flux generated by the induction coil into the process chamber. For this reason, the thickness of the dielectric material needs to be increased (i.e., in order to have vacuum force maintained), which results in sharply worsening ignitability and stability of the plasma as the distance between the induction coil and the top surface of the plasma is increased.
A third problem is concerned with the fact that the structure makes it difficult to have a grounding electrode set in parallel and opposite to a specimen, since a thick dielectric material has to be employed. Although an attempt has been made to increase the processing accuracy normally by applying a high-frequency bias to a specimen-holding stage in the case of an etcher or CVD, the processing tends to lack uniformity in the absence of such a grounding electrode positioned in parallel and opposite to the specimen; that is, in a case where the grounding electrode is positioned on the side wall of the chamber, the length of the high-frequency bias circuit portion which is allowed to pass through the plasma differs between the center of the wafer and the outer periphery of the wafer. Thus, the bias is unevenly applied to the wafer, and, especially when the wafer has a large diameter (8.fwdarw.12 inches), this problem becomes conspicuous.
A fourth problem originates from a variety of bad effects, including an abnormal discharge resulting from the high input impedance of the induction RF coil, which makes the power supply terminal have a high voltage, an unstable discharge resulting from sputtering and improper matching as electrostatically coupled components go on increasing, an impediment to the ignitability and so on.